﻿18 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol. 
  67 
  

  

  One 
  disturbing 
  factor 
  is 
  introduced 
  into 
  otherwise 
  consistent 
  data 
  

   by 
  the 
  Brazos 
  River 
  material 
  represented 
  by 
  the 
  anlaysis 
  given 
  in 
  

   columns 
  8 
  and 
  9 
  of 
  the 
  above 
  tabulation. 
  This 
  analysis, 
  which 
  

   differs 
  strikingly 
  from 
  all 
  the 
  others, 
  gives 
  the 
  formula 
  (Mg.Ca) 
  

   0. 
  5Fe 
  2 
  3 
  . 
  4Si0 
  2 
  . 
  10H 
  2 
  O. 
  This 
  sample 
  was 
  the 
  first 
  one 
  studied 
  

   and 
  about 
  1 
  gram 
  of 
  material 
  which 
  was 
  separated 
  for 
  analysis 
  was 
  

   estimated 
  to 
  contain 
  2 
  per 
  cent 
  of 
  augite 
  and 
  4 
  per 
  cent 
  of 
  plagio- 
  

   clase 
  of 
  the 
  composition 
  Ab 
  eo 
  An 
  40 
  . 
  The 
  figures 
  given 
  in 
  column 
  9 
  

   have 
  been 
  recalculated 
  after 
  correcting 
  for 
  these 
  impurities. 
  The 
  

   loss 
  of 
  water 
  below 
  110° 
  C. 
  was 
  determined 
  on 
  two 
  portions 
  yielding 
  

   4.40 
  and 
  4.48 
  per 
  cent, 
  respectively, 
  with 
  one 
  hours 
  heating 
  while 
  

   several 
  hours 
  continued 
  exposure 
  to 
  this 
  temperature 
  occasioned 
  no 
  

   further 
  loss. 
  The 
  dehydrated 
  powder 
  showed 
  no 
  change 
  in 
  any 
  of 
  

   its 
  optical 
  properties. 
  A 
  gain 
  of 
  2.28 
  per 
  cent 
  of 
  the 
  original 
  

   weight 
  was 
  acquired 
  by 
  a 
  dried 
  sample 
  upon 
  standing 
  overnight 
  in 
  a 
  

   dessicator 
  over 
  sulphuric 
  acid. 
  

  

  While 
  this 
  material 
  is 
  chemically 
  very 
  unlike 
  the 
  others 
  the 
  optical 
  

   properties, 
  other 
  than 
  refractive 
  index, 
  are 
  those 
  typical 
  of 
  idding- 
  

   site. 
  The 
  refractive 
  indices 
  are 
  very 
  high 
  as 
  would 
  be 
  expected 
  

   from 
  the 
  high 
  content 
  of 
  ferric 
  iron, 
  and 
  no 
  other 
  occurrences 
  of 
  

   such 
  high 
  refractive 
  index 
  have 
  been 
  recorded. 
  Optically 
  it 
  seems 
  

   to 
  be 
  a 
  true 
  iddingsite 
  but 
  until 
  similar 
  materials 
  from 
  other 
  locali- 
  

   ties 
  have 
  been 
  analyzed 
  no 
  conclusions 
  can 
  be 
  drawn 
  with 
  regard 
  

   to 
  its 
  relationship 
  to 
  the 
  ordinary 
  iddingsites 
  of 
  the 
  above 
  group. 
  

  

  SUMMARY 
  

  

  Iddingsite 
  is 
  a 
  red-brown 
  mineral 
  that 
  is 
  widespread, 
  and 
  often 
  

   an 
  abundant 
  mineral 
  in 
  basaltic 
  rocks. 
  

  

  It 
  occurs 
  as 
  cores 
  in 
  fresh 
  olivine, 
  as 
  rims 
  around 
  olivine, 
  or 
  where 
  

   cleavage 
  cracks 
  in 
  olivine 
  have 
  formed 
  a 
  locus 
  for 
  its 
  development. 
  

   Therefore 
  it 
  is 
  clearly 
  a 
  secondary 
  mineral 
  derived 
  from 
  olivine. 
  

  

  Iddingsite 
  is 
  not 
  confined 
  to 
  weathered 
  surfaces; 
  its 
  development 
  

   shows 
  no 
  proximity 
  to 
  joint 
  cracks 
  and 
  evidences 
  of 
  weathering 
  in 
  

   associated 
  minerals 
  arc 
  entirely 
  absent. 
  Normal 
  products 
  of 
  weather- 
  

   ing 
  such- 
  as 
  limonitic 
  pigment 
  are 
  absent, 
  but 
  spinels 
  (minerals 
  not 
  

   produced 
  by 
  weathering) 
  are 
  abundant 
  and 
  almost 
  invariable 
  as- 
  

   sociates. 
  Thus 
  it 
  is 
  concluded 
  that 
  iddingsite 
  is 
  not 
  a 
  product 
  of 
  

   ordinary 
  weathering 
  but 
  is 
  a 
  deuteric 
  mineral; 
  that 
  is, 
  it 
  is 
  the 
  

   result 
  of 
  metasomatic 
  processes 
  associated 
  with 
  the 
  later 
  stages 
  of 
  a 
  

   cooling 
  magma. 
  

  

  Iddingsite 
  does 
  not 
  commonly 
  occur 
  in 
  abyssal 
  rocks, 
  but 
  is 
  

   confined 
  to 
  extrusive 
  and 
  hypabyssal 
  rocks. 
  The 
  relations 
  indicate 
  

   that 
  it 
  is 
  formed 
  near 
  or 
  just 
  after 
  the 
  close 
  of 
  crystallization, 
  and 
  

   after 
  the 
  magma 
  came 
  to 
  rest. 
  The 
  factors 
  necessary 
  for 
  the 
  forma- 
  

   tion 
  are 
  an 
  olivine 
  of 
  suitable 
  composition, 
  a 
  concentration 
  of 
  

   mineralizers 
  (principally 
  water), 
  oxidizing 
  conditions 
  and 
  heat. 
  

  

  